1. Field of the Invention
The present invention relates to a communication system for a vehicle in which a plurality of control units (hereinafter referred to as ECUs) are connected to a communication signal line to perform control operations.
2. Description of the Prior Art
In a multiplex communication system for a vehicle, processing units (hereinafter referred to as CPUs) each having the same communication processing capacity are carried on each of the vehicles ECUs (such as the ECU of the door lock system for controlling the vehicle's door locks, the ECU of the engine system for controlling the vehicle's engine, etc.); thereby, communication between the ECU is realized.
However, in the above-mentioned system, there is an ECU which performs only input of a signal or control of an actuator, an ECU which also performs control of other control systems and the like, and the communication processing capacity required by each ECU differs depending on the type of control system, whereas CPUs each having the same communication processing capacity are carried on the ECUs so that efficient use of the CPUs' processing power is extremely bad.
3. Prior Work
The present inventor has developed a communication system for a vehicle as shown in FIG. 3. In FIG. 3, ECU 0 through ECU 5 are connected together using a communication signal line to establish mutual communication therebetween, and only the ECU 3 implements an arithmetic operation or communication processing function using a microprocessor or other such CPU as a central ECU, while other units such as ECU 0 to ECU 2, ECU 4 and ECU 5 are peripherals which implement a communication function and input and output signals without a need to implement the arithmetic operation function.
These peripheral ECUs are arranged at various places in the vehicle in order to perform control of door locks, the vehicle's engine, blinkers, lamps and the like. The peripheral ECUs monitor switch signals, detection signals and the like in their respective areas and provide signals to the communication signal line, control lamps, actuators and the like on the basis of data received from the communication signal line. A peripheral ECU circuit is shown in FIG. 2.
In FIG. 2, a communication IC 10 is driven by an oscillation element 11 via an XTAL 0 terminal and an XTAL 1 terminal. The communication IC 10 detects states of external switches SW 0 to SW 7 at signal input terminals IN 0 to IN 7 via an input circuit 15. States of the terminals IN 0 to IN 7 are outputted to the communication on signal line via a driver 12 from a TX terminal. Additionally, communication data inputted to an RX terminal of the communication IC 10 via a receiver 13 from the communication signal line is outputted front signal output terminals OUT 0 to OUT 7 of the communication IC 10.
The communication signal line is connected to a plurality of peripheral ECUs, (i.e., to ECU 0 to ECU 2, ECU 4, and ECU 5 as shown in FIG. 3) and also to the central ECU 3 in FIG. 3 implementing the arithmetic operation processing function, wherein signals inputted to each of the peripheral ECUs are received by ECU 3 via the communication signal line, and any necessary arithmetic operation processing functions are performed in the ECU 3 in accordance with the inputted information. The result of the processing performed by the ECU 3 is transmitted to the peripheral ECUs via the communication signal line, and the lamps or actuators connected thereto are controlled by the outputs of the peripheral ECU responsive to the processing results.
In order to make the peripheral ECUs which implement only inputting, outputting and communication functions low cost and compact, simple circuits must be used for the inputting circuit 15 and the outputting circuit 16. However, in the circuit shown in FIG. 2, when the ignition switch IG is turned ON to apply operative power to the system until relevant communication data is received from ECU 3 via the communication signal line, the initial states of the output terminals OUT 0 to OUT 7 cannot be established based solely on the internal information in each peripheral ECU. This means that any desired control cannot be performed in the initial state until the data is provided by the ECU 3. For example, in FIG. 2, if it is required that the lamps L0 to L2 are OFF and the lamp L3 is ON when the ignition switch IG is ON, if the output terminals OUT 0 to OUT 7 are high in their initial state, the lamps L0 and L1 are lit and L2 and L3 are turned off. On the other hand, if the output terminals OUT 0 to OUT 7 are low in their initial state, the lamps L0 and L1 are turned off and lamps L2 and L3 are lit. In other words, it is impossible to turn the lamps L0 to L2 off and turn only the lamp L3 on by setting the initial state of the output terminals OUT 0 to OUT 7. In such a case, it may be possible to add NOT circuits between the output transistors which drive the lamps and the output terminals of the communication IC 10. However, the internal state of the peripheral ECU is still not definite, so the circuits must be complicated to achieve the proper initial state of lamps L0 to L3.